For safety reasons, many industrial vehicles use spring-applied hydraulically-released brake systems (spring-applied brakes) to provide automatic fail-safe braking on loss of vehicle power or hydraulic fluid pressure. Such a system for wheel brakes is disclosed in U.S. Pat. No. 4,893,879 to Middelhoven, et al. Other such brake systems operate on a vehicle's driveline as an emergency brake.
The disadvantage of current vehicles equipped with spring-applied brakes is that such brakes prevent a disabled vehicle from easily being towed. This is a particular problem with vehicles used in mining, especially underground mining where access to a disabled vehicle is limited and the distance to a maintenance area may be great. This is particularly critical where remote-controlled, unmanned vehicles are used under unreinforced mine ceilings. Because workers are not permitted in such areas, repair and retrieval of a disabled vehicle may be impossible.
Some current systems for releasing spring-applied brakes to permit towing use an alternate backup pump and associated valves and power source on the vehicle to provide brake pressure in the event of a system failure and to provide a reserve to compensate for leakage. This type of backup system is costly to provide on every vehicle.
Another current system for releasing spring-applied brakes on a disabled vehicle is disclosed in U.S. Pat. No. 4,576,418 to Holtzinger, which discloses a brake release mechanism adapted to connect with a portable pump to provide sufficient hydraulic pressure to release the vehicle brakes. This requires a worker to approach the vehicle to make a connection.
Such current brake release systems require special equipment to be brought to the disabled vehicle, and increase the time needed to remove a disabled vehicle to permit its repair and to permit mining operations to begin. They also provide no means for braking the towed vehicle while it is being towed, an important safety consideration, especially in view of the typically heavy weight of such equipment.